Wave-signal amplitude-limiting system



Oct. 25, 1949. w. J. GRUEN WAVE-SIGNAL AMPLITUDE-LIMiTING SYSTEM Filed May 2, 1947 b m mEZmsZ o IN VEN TOR. WOLF J. GR UE N 0 52 5.5 a 65335 fizamzmfiza Y E N R O T T A Patented Oct. 25, 1949 WAVE-SIGNAL AMPLITUDE-LIMITING STEM Wolf Joachim Gruen, New York, N. Y., assignor to Hazeltine Research, Inc., Chicago, 111., a corporation of Illinois Application May 2, 1947, Serial No. 745,619

9 Claims. (Cl. 178-44) This invention relates to wave-signal amplitude-limiting systems and, particularly, to wavesignal amplitude-limiting systems which provide a limiting level that varies with the average amplitude of a wave signal applied to the system. Although the invention is not limited thereto, it has particular utility as a limiting system in a frequency-modulation wave-signal receiver and will be described in that connection.

Amplitude-limiting systems are frequently employed in frequency-modulation wave-signal receivers to reduce noise disturbances and distortion which otherwise may appear in the output circuit of the receiver due to undesired amplitude variations of the translated frequencymodulated wave signal. Some of these limiting systems perform their amplitude-limiting action only when the amplitude of the applied wave signal exceeds a predeterminad fixed level. Accordingly such systems are ineffective to remove undesired amplitude modulation from a wave signal having amplitude values which are less than the above-mentioned predetermined level. Qther limiting systems heretofore proposed are effective to provide a limiting level which varies with the average amplitude of the applied wave signal and therefore are able to remove to a considerable extent undesired amplitude variations appearing in the applied wave signal at both the lower amplitude and higher amplitude levels, thus ensuring less noise disturbance and distortion in the output signal of a frequency-modulation receiver employing such a limiting system. A system of the last-mentioned type is disclosed in the copending application of Harold A. Wheeler, Serial No. 693,268, filed August 27, 1946, entitled Wave-signal amplitude-limiting system, and assigned to the same assignee as the present invention.

The limiting system disclosed in the abovementioned copendlng application employs a series circuit including a rectifier device and a resistorcondenser network which is coupled effectively in parallel with a high-efiiciency or high-Q parallel-resonant circuit resonant at the mean frequency of the frequency-modulated wave signals applied thereto. As is well known, the Q of a resonant circuit is conveniently defined as the ratio of its inductive reactance to resistance. Peak rectification of the half cycles of but a single polarity of the applied wave signal takes place in the described series circuit due to the unidirectional conductivity characteristic of the rectifier device. The resistor in this series circuit, by loading the rectifier device, provides a substantial part of the total damping of the resonant circuit. This damping reduces the magnitude of the wave-signal potentials developed across the resonant circuit and thereby limits the amplitude of the wave signal at a level established by the average-amplitude value of the applied wave signal. Since the rectifier device of this arrangement provides a damping action only during the interval when it is conductive, the instantaneous damping action takes place primarily on alternate half cycles of the wave signal. Consequently, the rectifier device may be considered efiectively to control the peak amplitudes, and hence the limiting level of the wave signal, for only those alternate half cycles of the proper polarity to render the rectifier device conductive. The resonant circuit, however, tends to integrate this damping action over a complete cycle of the wave signal to an extent which is proportional to the magnitude of the Q of the resonant circuit. For best limitin operation with such a system, therefore, it is desirable to employ a resonant circuit having a high Q. For some applications, however, certain considerations such as space and cost may make it desirable to employ resonant circuits which do not have a high Q.

The type of limiting action afiorded by a system of the type disclosed in the above-mentioned application, while diminishing considerably the eiiect of undesired amplitude modulation in the frequency-modulated wave signals which are translated by the limiting system, does not quite assure the greatest possible reduction in unwanted amplitude modulation. It is well known that in most cases the limiting of, the amplitude of a wave signal results in the production of numerous harmonic-frequency components in the resultant wave signal. When significant harmonic voltages are thus produced, the frequency detector which is coupled to the limiting system may respond to these harmonic voltages to the detriment of its response to the desired fundamental-frequency components. The second-harmonic components, due to their closer proximity to the fundamental-frequency components in the case of the higher order harmonic components, are most troublesome. It may, therefore, in some applications, be desirable to avoid the production of second-harmonic components in the limiting system in order to ensure that the frequency detector has better response to the fundamental-frequency components of the wave signal'applied thereto. All

harmonic components, and particularly the unwanted second-harmonic components, are more pronounced when the Q of the parallel-resonant circuit associated with the limiting system is not high. This may be a disavantage for those applications wherein it is desirable to employ in association with the type of limiting system last described a resonant circuit which does not have a high Q.

It is an object of the present invention, therefore, to provide a new and improved wave-signal amplitude-limiting system having improved limiting characteristics particularly when used in association with a resonant circuit of relatively low Q.

It is another object of the invention to provide a new and improved wave-signal amplitudelimiting system which is effective to reduce the amplitudes of certain of the more troublesome harmonic-frequency components of the wave signals translated by the limiting system.

It is a further object of the invention to provide a new and improved wave-signal amplitudelimiting system which tends to eliminate the second and other even-order harmonic-frequency components which might otherwise appear in the wave signal translated by the limiting system.

It is another object of the invention to provide a new and improved wave-signal amplitude-limiting system of the variable-threshold type which automatically establishes a limiting level in accordance with the average amplitude of the frequency-modulated wave signal applied thereto and one which afiords improved rejection of undesired amplitude modulation.

In accordance with a particular form of the invention, a system for removing the undesired amplitude modulation of a frequency-modulated wave signal comprises a frequency-modulation wave-signal translating channel including a parallel-resonant circuit substantiall resonant at the center frequency of the pass band of the channel and having much less than critical damping. The system also includes a pair of rectifier devices and at least one pair of condensers connected to provide two series circuits each including an individual one of the rectifier devices and effectively including at least one of the condensers. Each of the series circuits is coupled effectively in parallel with the resonant circuit with the conductive direction of one of the series circuits opposite to that of the other, and each of the series circuits has in the conductive direction thereof at the aforesaid frequency an impedance much less than the impedance of the resonant circuit. The amplitude-limiting system includes at least one resistor efiectively connected in parallel with at least one of the condensers and having a value of resistance much greater than the conductive-direction impedance of either of the series circuits to effect peak rectification of each wave-signal half cycle. However, this value is sufiiciently small relative to the resonant-circuit impedance to cause by loading .the rectifier devices an average conductance providing a substantial part of the total dampin of the resonant circuit but much less than critical damping thereof. The condenser of each of the parallel-connected condensers and resistors has a value to provide with the resistor in parallel therewith a time constant both greater than the greatest radian period of the amplitude modulation to be removed from a wave signal translated by the channel and greater than the greatest radian period of the frequency modulation of the translated wave signal, whereby the bias potential developed across each parallel-connected condenser and resistor by rectification varies with the average amplitude of the translated wave signal to vary the amplitude level about which amplitude modulation is removed.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 is a circuit diagram, partly schematic, of a complete frequency-modulation wavesignal receiver which includes a wave-signal amplitude-limiting system embodying the present invention in a partic ular form; and Fig. 2 is a circuit diagram of a wave-signal amplitude-limiting system embodying the present invention in a modified form.

Referring now more particularly to Fig. 1 of the drawing, there is represented a complete frequency-modulation wave-signal receiver of somewhat conventional design which utilizes an amplitude-limiting system embodying one form of the present invention. In general, the receiver includes an oscillator-modulator I 0 having an in put circuit coupled to an antenna system H, l2 and having an output circuit coupled to an intermediate-frequency amplifier E3 of one or more stages. Connected in cascade with the amplifier iii, in the order named, are a wave-signal amplitude-limiting system H, more fully to be described hereinafter, a frequency detector l5 including a rectifier system [6, an audio-frequency amplifier ll of one or more stages, and a sound reproducer l8. The frequency detector I5 is preferably one having a linear frequency-response characteristic and may be of the type having side-tuned resonant input circuits as represented in Fig. 1.

An automatic-amplification-control or A. V. C. bias which is developed in unit I4 is applied in a conventional manner through a circuit designated as A. V. C. to the input circuits of one or more tubes of the oscillator-modulator l8 and the intermediate-frequency amplifier 13.

It will be understood that the various units just described may, with the exception of the wave-signal limiting system I 4;, be of conventional construction and operation, the details of which are well known in the art rendering further detailed description thereof unnecessary. Considering briefly the operation of the receiver as a whole, and neglecting for the moment the detailed operation of the wave-signal amplitudelimiting system l4 presently to be described, a desired frequency-modulated wave signal is selectively received and converted to a frequencymodulated intermediate-frequency wave signal in the oscillator-modulator H amplified in the intermediate-frequency amplifier l3, further amplified and limited in unit M to a limiting level varying in accordance with the average amplitude of the intermediate-frequency wave signal, and detected by the frequency detector 15, thereby to derive the audio-frequency modulation components thereof. The audio-frequency modulation components are, in turn, amplified in the audio-frequency amplifier ll and are reproduced by the sound reproducer l8 in a conventional manner. The A. V. C. bias developed in unit I4 is efiective to control the amplification of one or more of the units It and I3 to maintain the signal input to unit I4 within a rela v in detail hereinafter.

tively narrow range for a wide range of received signal intensities.

Referring now more particularly to the portion of the receiver embodying the present invention, the wave-signal amplitude-limiting system |4 comprises a wave-signal translating channel which conveniently includes one of the intermediate-frequency amplifier stages of the receiver. This channel is shown as including one such stage comprising a conventional vacuum-tube repeater 2|] having a tuned input circult which is connected through a transformer 2| to the tuned output circuit of the intermediate-frequency amplifier l3. The output circuit of the repeater 20 includes a parallel-resonant circuit 24 which is substantially resonant at the center frequency of the pass band of the intermediate-frequency channel and which has much less than critical damping. The parallelresonant circuit 24 includes the primary winding 26 of a transformer 21, one terminal of the winding being connected to the anode of the repeater 20 while the other terminal is connected to a source of potential indicated as +B and to ground through an intermediate-frequency bypass condenser 23, The parallel-resonant circuit 24 also includes in shunt to the winding 26 a condenser 30, shown in broken lines since it may be comprised in whole or in part of the inherent interelectrode and output-circuit capacitance of the repeater 20 and the capacitances of the rectifier devices 40 and 4| to be described Transformer 2! includes two secondary windings 32 and 33 which are tuned by condensers 34 and 35, respectively, to opposite sides of the center frequency of the intermediate-frequency wave-signal translating channel, thus forming two side-tuned resonant circuits 36 and 31. These tuned circuits are, in turn, coupled to individual rectifier circuits to provide a conventional side-tuned frequency detector I5.

The limiting system also includes a pair of rectifier devices 40 and 4|, preferably low-impedance thermionic diodes, and at least one pair of condensers 42, 43. The rectifier devices are connected to provide two series circuits each including an individual one of the rectifier devices and efiectively including at least one of the condensers last mentioned. The anode of the rectifier device 4|! is connected directly to the oathode of the device 4| while the grounded cathode of the former is connected to the anode of the last-mentioned rectifier device through the condenser 43. Condenser 43 has a relatively large value of capacitance. The condenser 42, which has a smaller value of capacitance than that of the condenser 43, is connected between the anodes of tubes 40 and 20. It will be seen, therefore, that in the particular arrangement shown the pair of rectifier devices and the pair of condensers form two series circuits, one circuit comprising the condenser 42 and the rectifier 40 while the other includes the rectifier device 4| and both of the condensers 42 and 43. It will also be manifest that each of the described series circuits is coupled effectively in parallel with the resonant circuit 24 with the conductive direction of one of the series circuits opposite to that of the other. In accord ance with the present invention, the circuit parameters of the limiting system are selected that the series circuits have in their conductive directions at the center frequency of the pass band of the intermediate-frequency channel an impedance much less than the impedance'of the resonant circuit.

The amplitude-limiting system also includes at least one resistor effectively connected in parallel with at least one of the condensers in the above-mentioned series circuits and having a value of resistance much greater than the conductive-direction impedance of either of the described series circuits to effect peak rectification of each wave-signal half cycle. This element comprises a resistor 45 which is connected in parallel with the relatively large-capacitance condenser 43. Alternatively the resistor 45 may comprise two portions preferably having equal values, each portion being connected in parallel with an individual one of the rectifier devices and 4| and thus effectively in parallel with the condenser 43. The value of the resistor 45, while being greater than the conductive-direction impedance of either series circuit, is sufficiently small relative to the impedance of the resonant circuit 24 to cause by loading the rectifier devices 40 and 4| and average conductance which provides a substantial portion of the total damping of the resonant circuit but much less than critical damping thereof.

The condenser of each of the parallel-connected condensers and resistors, namely the condenser 43 since there is only one such combination in the described rectifier arrangement, has a value to provide with the resistor 45 in parallel therewith a time constant which is greater than the greatest radian period of the amplitude modulation to be removed from a wave signal translated by the intermediate-frequency channel. For this value of the condenser or condensers, the bias potential developed across each such paralel-connected resistor-condenser combination by peak rectification varies with the average-amplitude value of the translated wave signal to vary the limiting level of the system in accordance therewith yet has a substantially constant value for the undesired amplitude modulation to effect amplitude limiting by the system at the aforesaid level.

The described arrangement of the rectifier devices 40, 4| and condensers 4'2, 43 provides a voltage-doubling rectifier system in which the unidirectional voltage developed across the condenser 43 has a value equal to twice the average peak amplitude of the intermediate-frequency wave signal. Since this value of unidirectional voltage may be too large to provide correct A. V. C. action, the A. V. C. circuit may be connected as indicated to an intermediate point on the resistor 45 for supplying a portion only of the bias developed across the condenser43 to units l0 and I3 to control the gain thereof as previously described.

Considering now the operation of the wavesignal amplitude-limiting system just described, the wave signals developed across the resonant circuit 24 are also applied to the two series cirouits of the rectifier arrangement 46. Assuming first that the instantaneous polarity of the wave signal is such that the ungrounded terminal of the resonant circuit 24 is positive, the positive half cycle of the developed signal is effective to render only the device 40 conductive, thus producing an increase in the charge on the condenser 42. The unidirectional potential developed across the condenser 42 from the increased charge is substantially equal in magnitude to the positive peak value of the frequency-modulated Wave signal. On the next or negative half cycle, the

device 40 ceases to conduct but the rectifier deanswer vice-'41 then has. proper polarity to berrendered conductive. There is applied at this time-(to the series :circuit comprising the rectifier device *4! and the :condenser 43 an instantaneous peak potential comprising the potential appearing .across the winding 26 plus the increased potential earlier developed across the condenser 42. These potentia'ls are in series-aiding .relationshipiso that sub stantially the peak-to-peak potential of the wave signaldeveloped :across the parallel-resonant circuit 24in a complete wave-signal cycle israpp'lied to the're'ctisfier device 4d and condenser '43. Condenser 43 is therefore charged to .a potential value which is substantiallytwice that developed across the condenser 42 :on the preceding half cycle,=and the condenser 42 is simultaneously discharged. The described peak-to-peak rectification operation is repeated for successive wavesignal cycles.

The amount of damping aiforded by the rectifier arrangement-46 isdetermined by the amount of .energy taken from the peak portion of the wave-signal oscillation during each half cycle thereof. This is, in turn, determined by the amountwof energy discharged during each cycle from the condenser-43 through the-resistor d5, atid'the energy discharge during each cycle is controlled by the time constant of the'resistorcondenser combination -45 43. Y Peak-rectification action-requires that the two series circuits including the rectifier devices 40 and 4 shall include no additional impedance sufficient substantially to reduce the peak current which flows through the rectifiendevices 40 and M during their respective conductive periods. The resonant circuit 24, 'per se', is one of relatively small damping; so thatpeak rectification of each half cycle by devices ll and 41, while providing a substantial-portion-of the total damping of the resonant-circuit, affords much less than critical damping thereof. This damping reduces the magnitude of "the wavesignal potential developed across the resonant c'ircuitt24and therefore affords a' limiting action.

'When'tl'ie condenser 4'3 :and't-he-resistor 45 have values as mentioned above, the described loading of the rectifier devices 41'] -and4l' permits the amplitude f the'wayasignal potentials developed across the resonant circuit 24 to rise and fall slowly with the variations of 'the'wave-signal average amplitude. "During slow variations of the wave-signal amplitude, "the condenser 43 has time to charge or to discharge slowly. However, the time constant of the resistor-condenser combination d5, 43 is-substan'tially greater than the greatest radian period ofthe amplitude modulation'which is to be removed-from the wvave signal. This :time .constant prevents the condenser *4'3 from charging =or discharging at as rapid arate as the undesired amplitude modulation sothat the condenser voltage-acts like a fixed bias voltage 'for'such modulation. As a result, the 'bias potential adjusts or varies the-limiting 'level of the limiting system'i inin accordance withthe averageamplitude of the translated intermediatefrequency .wave signal, yethas a substantially constant .value 'withrespcct to the undesired amplitudemodulation-to be removed and' thus'eifec'ts amplitude limiting at this ievel in'thesystem M.

A portion of the :bias potential, when-applied as an .AFV. C. .bias to-units l ll and 13 from-an intermediate potentialzpointlon the resistor, is 'eiiective to limit the averageaamplitude variations of the wave signal applied to the :limiting system: 4-4.

As .a result of the ,peak .:rectification by the rectifierwarrangement 4B .of: each-half .zcyclerni the-signal applied to the resonant circuitdL-instead of every other half cycle as with prior limiting arrangements :which employ .a-.'resonant coircuit. the effective conductance which dam ethe parallel-resonant circuit provides a morei-uniiorm damping/action. This, in turn, permits the use in the limiting system of a parallel-resonant circuit which need not have a particularly high 'Q. The peak-to-peak rectification afiorded by thearrangement '46 producesa symmetrical conduction on both half cycles of thefrequencymodulated wave signal applied to the resonant circuit '24. Rectification of both half cycles :of the applied wave signal also tends to cancer-out the troublesome second and other even-order harmonic components. It has been determined that a significant increase in amplitude-modulation rejection is provided. by the instant system over that which is obtained with limiting systems which peak rectify only the alternate ha'lf cycles of the applied wave signal. In the higher wavesignal average-amplitude levels, this improvement may be as much as four to one.

The following circuit constants are given-'for an embodiment of the invention of the type'represented in Fig. 1:

Resistor 27.0 kilohms Condenser '42 100 micromicrofarads Condenser 43 0.25 microfarad Tube 2!; Type 6AU6 Tubes 48 and 4| 6AL5 (Duplex .diode) Winding 26 -30 turns N0. 30.D..S. C.

wire close wound on diameter winding form. Intermediate frequency". 10.7 megacycles Referring now to Fig. 2 of the drawingythere .is represented schematically a wave-signal amplitude-limiting system embodying the 1 present invention .in a modified form which is generally similar to that represented in Fig. 1, similar *elem'ents being designated by the same reference numerals primed. The output circuit of arepeater 20' includes a parallel-resonant circuit 24. Theanode of'a rectifier device 40 is connected to the anode of repeater 20' through a coupling condenser 52 while the cathode of the device is connected to ground. The cathode' of a rectifier device 4| is also connected -to the anode of the repeater 20 through a coupling-condenser 53 andthe anode-of this device is grounded. Rectifier device 40 has connected between the terminals thereof a series circuit comprising an intermediate-frequency choke 55 andthe parallel combination of a resistor 56 and a condenser: 51. The A. V. C. bias 'potentialfor stages in the wavesignal translating channel preceding the parallel-resonant circuit 24' is taken from thejunction of the choke 55 and the -resistor56. The rectifier device 4|" has connected between itste'rminals a, series circuit comprising a choke 55 and a parallel resistor-condenser combination 68, E1. The network including the condenser 52, the rectifier device 40, the choke the resistor 56, and the condenser 51-comprises a first'series circuit "while the network including "the corresponding elements-'53, 41",65, 6'6, and Bloomprises "a second series circuit, each Of"WhiCh.'iIlcludes an individualone of the'rectifi'er 'devices and effectively includes at least one of the condensers 5-I- and 61. The descr'ibedseries circuits have-substantially identical characteristics and are coupled in parallel with the resonant circuit 24'. Each ofthe described series circuits has,

in its respective conductive direction and at the center frequency of the intermediate-frequency pass band, an impedance which is much less than the impedance of the resonant circuit. In the conductive direction of the first series circuit this impedance comprises that of the condenser 52 and that of the rectifier device 40 while in the conductive direction of the second series circuit this impedance comprises that of the rectifier device M and that of the condenser 53. Each of the condensers 51 and 61 has a value to provide with the particular resistor in parallel therewith a time constant which is greater than the greatest radian period of the amplitude modulation to be removed from a wave signal translated by the system.

The resistors 56 and 66 may be connected if desired in parallel with their corresponding rectifiers 40' and 4| instead of being connected in parallel with the condensers 51 and 61, as shown. Other variations of the limiting system represented in Fig. 2 will be manifest. For example the condensers 51 and 61 may, if desired, be omitted. In this case the condensers 52 and 53 are selected with reference to the resistors 56 and 66, in the manner previously explained in detail, to provide the required time constants. The chokes 55 and 65 may also be omitted from the last-mentioned arrangement. The resistors 56 and 66 then load the tuned circuit 24' .and are effective to decrease the range of the damping effect which may be obtained with the devices 40' and 4|. When automatic volume control is desired, the connection therefor must be isolated for intermediate-frequency wave signals from the resistor 56.

The operation of the Fig. 2 amplitude-limiting system is quite similar to that of Fig. 1. Peak rectification of wave-signal half cycles of positive polarity are effected by a series circuit including the rectifier device 40' while peak rectification of wave signal half cycles of negative polarity are effected by the other series circuit including the rectifier device 4|. Thus the arrangement including the rectifier devices All and 4| tends to limit on individual ones of the positive and the negative peak amplitudes of the applied wave signal, as distinguished from the conjoint peak-to-peak action as in the Fig. 1 system. Otherwise the operation of the two limiting systems is essentially the same. Symmetrical limiting of the frequency-modulated wave signal which is translated by the amplitude-limiting system I4 results, and the translated signal is substantially free from undesired amplitude modulation. The entire unidirectional potential developed across the condenser 51 may be employed in the usual manner as an A. V. C. bias.

It will be apparent from the foregoing description that a wave-signal translating system embodying the present invention is effective, by substantially eliminating the second and higher order even-harmonic components from the translated wave signal and by afiording a symmetrical wavesignal limiting action, to provide an output signal which is substantially free from undesired amplitude modulation. The amplitude-limiting systern of the present invention has the advantage of permitting the use therein of a resonant circuit which does not require a high Q. Furthermore the amplitude-limiting system of the instant invention provides good amplitude limiting at any average-amplitude value of an applied wave signal within a substantial range of such amplitude values.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A system for removing the undesired amplitude modulation of a frequency-modulated wave signal comprising: a frequency-modulation wave-signal translating channel including a parallel-resonant circuit substantially resonant at the center frequency of the pass band of said channel and having much less than critical damping; a pair of rectifier devices and at least one pair of condensers connected to provide two series circuits each including an individual one of said rectifier devices and effectively including at least one of said condensers, each of said series circuits being coupled efiectively in parallel With said resonant circuit with the conductive direction of one of said series circuits opposite to that of the other and each of said series circuits having in the conductive direction thereof at said frequency an impedance much less than the impedance of said resonant circuit; and at least one resistor effectively connected in parallel with at least one of said condensers and having a value of resistance much greater said conductive-direction impedance of either of said series circuits to effect peak rectification of each wave-signal half cycle yet sufficiently small relative to said resonant-circuit impedance to cause by loading said rectifier devices an average conductance providing a substantial part of the total damping of said resonant circuit but much less than critical damping thereof; the condenser of each of said parallel-connected condensers and resistors having a valve to provide with the resistor in parallel therewith a time constant both greater than the greatest radian period of the amplitude modulation to be removed from a Wave signal translated by said channel and greater than the greatest radian period of the frequency modulation of the translated wave signal, whereby the bias potential developed across said each parallelconnected condenser and resistor by rectification varies with the average amplitude of said translated Wave signal to vary the amplitude level about which amplitude modulation is removed.

2. A system for removing the undesired amplitude modulation of a frequency-modulated wave signal comprising: a frequency modulation Wavesignal translating channel including a parallelresonant circuit substantially resonant at the center frequency of the pass band of said channel and having much less than critical damping; a pair of low-impedance rectifier devices and at least one pair of condensers connected to provide two series circuits each including an individual one of said rectifier devices and effectively including at least one of said condensers, each of said series circuits being coupled effectively in parallel with said resonant circuit with the com ductive direction of one of said series circuits opposite to that of the other and each of said series circuits having in the conductive direction thereof at said frequency an impedance much less than the impedance of said resonant circuit; and at least one resistor eifectively connected in parallel with at least one of said condensers and having a value of resistance much greater than said age amplitude of said translated wave signal to vary the amplitude level about which amplitude modulation is removed; and means connected to an intermediate potential point on said resistor for utilizing a portion of the bias potential developed across said condenser to control the gain of said channel to reduce the range of averageamplitude variations of said translated wave signal.

6. A system for removing the undesired amplitude modulation of a frequency-modulated wave signal comprising: a frequency modulation wavesignal translating channel including a parallelresonant circuit substantially resonant at the center frequency of the pass band of said channel and having much less than critical damping; a pair of rectifier devices and at least one pair of condensers connected to provide two series circuits each including an individual one of said rectifier devices and effectively including at least H one of said condensers, each of said series circuits being capacitively coupled by at least one of said condensers effectively in parallel with said resonant circuit with the conductive direction of one of said series circuits opposite to that of the other and each of said series circuits having in the conductive direction thereof at said frequency an impedance much less than the impedance of said resonant circuit; and at least one resistor effectively connected in parallel with at least one of said condensers and having a value of resistance much greater than said conductive-direction impedance of either of said series circuits to effect peak rectification of each wave-signal half cycle yet sufiiciently small relative to said resonant-circuit impedance to cause by loading said rectifier devices an average conductance providing a substantial part of the total dampin of said resonant circuit but much less than critical damping thereof; the condenser of each of said parallel-connected condensers and resistors having a value to provide with the resistor in parallel therewith a time constant both greater than the greatest radian period of the amplitude modulation to be removed from a wave signal translated by said channel and greater than the greatest radian period of the frequenc modulation of the translated wave signal, whereby the bias potential developed across said each parallel-connected condenser and resistor by rectification varies with the average amplitude of said translated wave Signal to vary the amplitude level about which amplitude modulation is removed.

'7. A system for removin the undesired amplitude modulation of a frequency-modulated wave signal comprising: a frequency-modulation wavesignal translating channel including a parallelresonant circuit substantially resonant at the center frequency of the pass band of said channel and having much less than critical damping; a pair of rectifier devices and at least one pair of condensers connected to provide two series circuits each including an individual one of said rectifier devices and effectively including at least one of said condensers, said condensers having unequal values of capacitance and the one of said condensers of smaller capacitance being common to said pair of series circuits, each of said series circuits being coupled eifectively in parallel with said resonant circuit with the conductive direction of one of said series circuits opposite to that of the other and each of said series circuits having in the conductive direction thereof at said frequency an impedance much less than the impedance of said resonant circuit; and a resistor Ii U effectively connected in parallel with the other of said condensers and having a value of resistance much greater than said conductive-direction impedance of either of said series circuits to effect peak rectification of each wave-signal half cycle yet sufficiently small relative to said resonantcircuit impedance to cause by loading said rectifier devices an average conductance providing a substantial part of the total damping of said resonant circuit but much less than critical damping thereof; said other condenser having a value to provide with said resistor in parallel therewith a time constant both greater than the greatest radian period of the amplitude modulation to be removed from a wave signal translated by said channel and greater than the greatest radian period of the frequency modulation of the translated wave signal, whereby the bias potential developed across said parallehconnected condenser and resistor by rectification varies with the average amplitude of said translated wave signal to vary the amplitude level about which amplitude modulation is removed.

8. A system for removing the undesired amplitude modulation of a frequency-modulated wave signal comprising: a frequency-modulation wavesignal translating channel including a parallelresonant circuit substantially resonant at the center frequency of the pass band of said channel and having much less than critical damping; a pair of rectifier devices and at least one pair of condensers connected to provide two series circuits each including an individual one of said rectifier devices and including at least one of said condensers, each of said series circuits bein coupled effectively in parallel with said resonant circuit with the conductive direction of one of said series circuits opposite to that of the other and each of said series circuits having in the conductive direction thereof at said frequency an impedance much less than the impedance of said resonant circuit; and a pair of resistors, each effectively connected in parallel with an individual one of said condensers and having a value of resistance much greater than said conductivedirection impedance of either of said series circuits, to effect peak rectification of each wavesignal half cycle, yet the value of each of said resistors being sufficientl small relative to said resonant-circuit impedance to cause by loading said rectifier devices on alternate wave-signal half cycles an average conductance providing a substantial part of the total damping of said resonant circuit but much less than critical damping thereof; the condenser of each of said parallel-connected condensers and resistors having a value to provide with the resistor in parallel therewith a time constant both greater than the greatest radian period of the amplitude modulation to be removed from a wave signal translated by said channel and greater than the greatest radian period of the frequency modulation of the translated wave signal, whereby the bias potential developed across said each parallel-connected condenser and resistor by rectification varies with the average amplitude of said translated wave signal to vary the amplitude level about which amplitude modulation is removed.

9. A system for removing the undesired amplitude modulation of a frequency-modulated wave signal comprising: a frequency-modulation wavesignal translating channel including a parallelresonant circuit substantially resonant at the center frequency of the pass band of said channel and having much less than critical damping;

characteristics, each ofi saidseries circuitsin-.

eluding an individual one of said rectifier devices and at least one of said condensers. and bein coupled effectivelyin para-llelwith said resonant circuit with, the conductive direction of one-of said series circui-ts: opposite to that. of the other and: having, inthe conductive directionthereof atsaid= frequency an impedance muchlqless than the impedance of said resonant circuit; and a pair of resistors, eachv eiiectively eonnectedin parallel. with an individual one of said condensers and having a value ofresistance', which is much greater than said conductiveedirection' impedance of, either of said series circuits, to efiect identical peak rectification of-zeach wave-signalhali cycle, yet the value of eachof said resistors. bein sufliciently small. relative to said resonant-circuit impedance to cause by loading said rectifier devices on alternate wave-signal half cycles an average conductance providing a-substantialpart of the: total damping of. said; resonant circuit but much less than critical, damping thereof; the condenser. of each; of said parallel-connected condensers and resistorsrhaving a. values-to. provide with the resistor in paralleltherewith a time con.- tan-tboth. greater than the-greatest radian period of the amplitude modulation to be removed. from a Wave signal translated by said channel and greaterthancthe greatest radian period of-the frequency modulation of the translated wave-Sig: na-l, whereby the bias, potential. developed across said each parallel-connected condenser and. resister-b rectification varies with the. average amplitude of said-translatedwave signal to vary the amplitude level about which amplitud-elmodulationis removed.

WOLF J CACHE/I GRUEN..

REFERENCES CITED The following references are of record in the file of this patent? UNITED STATES PATENTS Number Name Date 2,224,794 Montgomery Dec; 10; 1940 2,263,165 Dallos' Nov. 18,- 194i 2,28%);0'44 Morris June 2; 1-942 2,340,429 Rankin Feb. 1; 1944 

